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4. (a) (5 marks) Consider the following time series model for {y: },: y1 = ULITEL+ AEL- 1, where , is i.i.d with mean zero and variance o', for t - 1, ..., . Let yo - 0. Demon strate that y is non-stationary unless A - -1. In your answer, clearly provide the conditions for a covariance stationary process. Hint: Apply recursive substitution to express y in terms of current and lagged errors. (b) (3 marks) Briefly discuss the problem of applying the Dickey Fuller test when testing for a unit root when the model of a time series , is given by: where the error term > exhibits autocorrelation. Clearly state what the null, alternative hypothesis, and the test statistics are for your test